Manufacture of materials from graphite particles

ABSTRACT

A method of manufacturing flexible sheets of expanded graphite material from recycled materials, comprising providing source materials in the form of flexible sheets of expanded graphite; comminuting the source materials into particles; re-expending the particles; and preparing a mat from the re-expanded particles. Also described herein is a process of manufacturing a graphite material comprising grinding a cured resin impregnated graphite material into particles; removing at least part of the resin from the particles; and expanding the resin removed particles.

TECHNICAL FIELD

A method is provided for manufacturing flexible sheets or mats ofexpanded graphite material from recycled materials. The manufacturingmethod of the present invention may be used to manufacture flexiblesheets or mats of expanded graphite material that are suitable for usein the manufacture of components in a proton exchange membrane fuelcell, such as a flow field plate or an electrode.

BACKGROUND ART

An ion exchange membrane fuel cell, more specifically a proton exchangemembrane (PEM) fuel cell, produces electricity through the chemicalreaction of hydrogen and oxygen in the air. Within the fuel cell,electrodes denoted as anode and cathode surround a polymer electrolyteto form what is generally referred to as a membrane electrode assembly,or MEA. Oftentimes, the electrodes also function as the gas diffusionlayer (or GDL) of the fuel cell. A catalyst material stimulates hydrogenmolecules to split into hydrogen atoms and then, at the membrane, theatoms each split into a proton and an electron. The electrons areutilized as electrical energy. The protons migrate through theelectrolyte and combine with oxygen and electrons to form water.

A PEM fuel cell includes a membrane electrode assembly sandwichedbetween two graphite flow field plates. Conventionally, the membraneelectrode assembly consists of random-oriented carbon fiber paperelectrodes (anode and cathode) with a thin layer of a catalyst material,particularly platinum or a platinum group metal coated on isotropiccarbon particles, such as lamp black, bonded to either side of a protonexchange membrane disposed between the electrodes. In operation,hydrogen flows through channels in one of the flow field plates to theanode, where the catalyst promotes its separation into hydrogen atomsand thereafter into protons that pass through the membrane and electronsthat flow through an external load. Air flows through the channels inthe other flow field plate to the cathode, where the oxygen in the airis separated into oxygen atoms, which joins with the protons through theproton exchange membrane and the electrons through the circuit, andcombine to form water. Since the membrane is an insulator, the electronstravel through an external circuit in which the electricity is utilized,and join with protons at the cathode. An air stream on the cathode sideis one mechanism by which the water formed by combination of thehydrogen and oxygen is removed. Combinations of such fuel cells are usedin a fuel cell stack to provide the desired voltage.

The flow field plates may be flexible graphite sheets as describedherein that are deformed into a shape (e.g., by embossing, stamping,molding, or a calender roll) that has a continuous reactant flow channelwith an inlet and an outlet. The inlet is connected to a source of fuelin the case of an anode flow field plate, or a source of oxidant in thecase of a cathode flow field plate. When assembled in a fuel cell stack,each flow field plate functions as a current collector.

Electrodes may be formed by providing a graphite sheet as describedherein and providing the sheet with channels, which are preferablysmooth-sided, and which pass between the parallel, opposed surfaces ofthe flexible graphite sheet and are separated by walls of compressedexpandable graphite. It is the walls of the flexible graphite sheet thatactually abut the ion exchange membrane, when the inventive flexiblegraphite sheet functions as an electrode in an electrochemical fuelcell.

The channels are formed in the flexible graphite sheet at a plurality oflocations by mechanical impact. Thus, a pattern of channels is formed inthe flexible graphite sheet. That pattern can be devised in order tocontrol, optimize or maximize fluid flow through the channels, asdesired. For instance, the pattern formed in the flexible graphite sheetcan comprise selective placement of the channels, as described, or itcan comprise variations in channel density or channel shape in order to,for instance, equalize fluid pressure along the surface of the electrodewhen in use, as well as for other purposes which would be apparent tothe skilled artisan.

The impact force is preferably delivered using a patterned roller,suitably controlled to provide well-formed perforations in the graphitesheet. In the course of impacting the flexible graphite sheet to formchannels, graphite is displaced within the sheet to disrupt and deformthe parallel orientation of the expanded graphite particles. In effectthe displaced graphite is being “die-molded” by the sides of adjacentprotrusions and the smooth surface of the roller. This can reduce theanisotropy in the flexible graphite sheet and thus increase theelectrical and thermal conductivity of the sheet in the directiontransverse to the opposed surfaces. A similar effect is achieved withfrusto-conical and parallel-sided peg-shaped flat-ended protrusions.

Graphites are made up of layer planes of hexagonal arrays or networks ofcarbon atoms. These layer planes of hexagonally arranged carbon atomsare substantially flat and are oriented or ordered so as to besubstantially parallel and equidistant to one another. The substantiallyflat, parallel equidistant sheets or layers of carbon atoms, usuallyreferred to as graphene layers or basal planes, are linked or bondedtogether and groups thereof are arranged in crystallites. Highly orderedgraphites consist of crystallites of considerable size: the crystallitesbeing highly aligned or oriented with respect to each other and havingwell ordered carbon layers. In other words, highly ordered graphiteshave a high degree of preferred crystallite orientation. It should benoted that graphites possess anisotropic structures and thus exhibit orpossess many properties that are highly directional e.g. thermal andelectrical conductivity and fluid diffusion.

Briefly, graphites may be characterized as laminated structures ofcarbon, that is, structures consisting of superposed layers or laminaeof carbon atoms joined together by weak van der Waals forces. Inconsidering the graphite structure, two axes or directions are usuallynoted, to wit, the “c” axis or direction and the “a” axes or directions.For simplicity, the “c” axis or direction may be considered as thedirection perpendicular to the carbon layers. The “a” axes or directionsmay be considered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. The graphites suitablefor manufacturing flexible graphite sheets possess a very high degree oforientation.

As noted above, the bonding forces holding the parallel layers of carbonatoms together are only weak van der Waals forces. Natural graphites canbe treated so that the spacing between the superposed carbon layers orlaminae can be appreciably opened up so as to provide a marked expansionin the direction perpendicular to the layers, that is, in the “c”direction, and thus form an expanded or intumesced graphite structure inwhich the laminar character of the carbon layers is substantiallyretained.

Graphite flake which has been greatly expanded and more particularlyexpanded so as to have a final thickness or “c” direction dimensionwhich is as much as about 80 or more times the original “c” directiondimension can be formed without the use of a binder into cohesive orintegrated sheets of expanded graphite, e.g. webs, papers, strips,tapes, foils, mats or the like (typically referred to as “flexiblegraphite”). The formation of graphite particles which have been expandedto have a final thickness or “c” dimension which is as much as about 80times or more the original “c” direction dimension into integratedflexible sheets by compression, without the use of any binding material,is believed to be possible due to the mechanical interlocking, orcohesion, which is achieved between the voluminously expanded graphiteparticles.

In addition to flexibility, the sheet material, as noted above, has alsobeen found to possess a high degree of anisotropy with respect tothermal and electrical conductivity and fluid diffusion, comparable tothe natural graphite starting material due to orientation of theexpanded graphite particles and graphite layers substantially parallelto the opposed faces of the sheet resulting from very high compression,e.g. roll pressing. Sheet material thus produced has excellentflexibility, good strength and a very high degree of orientation.

Briefly, the process of producing flexible, binderless anisotropicgraphite sheet material, e.g. web, paper, strip, tape, foil, mat, or thelike, comprises compressing or compacting under a predetermined load andin the absence of a binder, expanded graphite particles which have a “c”direction dimension which is as much as about 80 or more times that ofthe original particles so as to form a substantially flat, flexible,integrated graphite sheet. The expanded graphite particles thatgenerally are worm-like or vermiform in appearance, once compressed,will maintain the compression set and alignment with the opposed majorsurfaces of the sheet. The density and thickness of the sheet materialcan be varied by controlling the degree of compression. The density ofthe sheet material can be within the range of from about 0.04 g/cc toabout 2.0 g/cc. The flexible graphite sheet material exhibits anappreciable degree of anisotropy due to the alignment of graphiteparticles parallel to the major opposed, parallel surfaces of the sheet,with the degree of anisotropy increasing upon roll pressing of the sheetmaterial to increased density. In roll pressed anisotropic sheetmaterial, the thickness, i.e. the direction perpendicular to theopposed, parallel sheet surfaces comprises the “c” direction and thedirections ranging along the length and width, i.e. along or parallel tothe opposed, major surfaces comprises the “a” directions and the thermaland electrical properties of the sheet are very different, by orders ofmagnitude, for the “c” and “a” directions.

Methods of manufacturing articles from graphite particles have beenproposed. For example, U.S. Pat. No. 5,882,570 to Hayward discloses amethod of grinding flexible unimpregnated graphite foil to a smallparticle size, thermally shocking the particles to expand them, mixingthe expanded graphite with a thermoset phenolic resin, injection moldingthe mixture to form low density blocks or other shapes, then heattreating the blocks to thermoset the material. The resulting blocks maybe used as insulating material in a furnace or the like.

WO 00/54953 and U.S. Pat. No. 6,217,800, both to Hayward furtherdescribe processes related to those of U.S. Pat. No. 5,882,570.

The Hayward processes are very limited in the scope of the sourcematerials they use, and the type of end products they can produce.Hayward uses only unimpregnated graphite source materials, and hisfinished products are only formed by mixing the graphite powder withlarge proportions of resin and injection molding the mixture to formarticles which are then thermoset.

Accordingly, there is a continuing need in the art for improvedprocesses for producing flexible graphite sheets or products fromvarious types of graphite materials, including those which are alreadyresin impregnated, and for manufacture of more broadly useful productsfrom those materials. Such improved processes are provided by thepresent invention.

DISCLOSURE OF THE INVENTION

As stated above, the present invention provides a method ofmanufacturing flexible sheets or mats of expanded graphite material. Themats manufactured by the process of the present invention areparticularly useful in the manufacture of components in a PEM fuel cell,including electrodes and flow field plates.

In the production and use of flexible graphite sheets, scrap materialmay be generated. For example, in the production of flow field plates, aflexible graphite sheet may be shaped, impregnated with a resin, andafter impregnation, cured. During this process, scrap flexible graphitesheet material may be produced before impregnation, after impregnationand before curing, and after impregnation and after curing. The scrapflexible graphite sheet material used before impregnation is describedherein as regrind material or virgin regrind material. Sheet materialproduced after impregnation and before curing is described herein asuncured impregnated scrap (production scrap). The material producedafter impregnation and after curing is described herein as cured regrind(regrind scrap). Furthermore, the present invention may use, as a sourcematerial, sheet material produced from recycled used material such asrecycled fuel cell flow field plates.

Using the methods of the present invention, one can regrind the abovesheet material and recycle the same into flexible graphite mats that maybe further processed for use, e.g., as a material which can be formedinto a component in a fuel cell.

The method of the present invention is advantageous because it providesa beneficial re-use of flexible graphic sheet material such as theuncured impregnated scrap and cured impregnated scrap used in theproduction of, for example, flow field plates. The present inventionprovides an advantageous use for such material and decreases disposalcosts.

Specifically, one embodiment of the present invention is a method ofmanufacturing flexible sheets of expanded graphite material fromrecycled materials, comprising providing source materials in the form offlexible sheets of expanded graphite; comminuting the source materialsinto particles; re-expanding the particles; and preparing a mat from there-expanded particles.

Another embodiment of the present invention is a process ofmanufacturing a graphite material comprising grinding a cured resinimpregnated graphite material into particles; removing at least part ofthe resin from the particles; and expanding the resin removed particles.

It is an object of the present invention to provide a method forpreparing graphite mats that can be manufactured from recycledmaterials.

Yet another object of the present invention is to provide materialsuitable for the construction of a component of a fuel cell manufacturedusing expanded graphite material from recycled materials.

Still another object of the present invention is to provide a method formanufacturing flexible sheets of graphite material from, as a sourcematerial, unimpregnated graphite sheet material, uncured resinimpregnated graphite sheet material, and cured resin impregnatedgraphite sheet material.

Another object of the present invention is to provide a method ofmanufacturing flexible sheets of expanded graphite material from, as asource material, used, recycled, graphite material such as flow fieldplates and electrodes.

Another object of the present invention is to provide a process formanufacturing graphite material comprising: recycling resin impregnatedgraphite material and removing the resin from the recycled graphitematerial. The process of this embodiment includes removing at least partof the resin from the source material and re-expanding the resin removedparticles.

Other and further objects, features, and advantages would be readilyapparent to those skilled in the art, upon a reading of the followingdisclosure when taken in conjunction with the accompanying drawings.

FIG. 1 is a flow chart showing an embodiment of the present inventionwherein a flexible graphite sheet is manufactured.

FIG. 2 is a schematic drawing of a process for producing flexiblegraphite sheets which may be used as source materials in the presentinvention.

FIGS. 3-5 are charts showing a thermogravimetric analysis demonstratingthe temperatures at which resin may be removed from the source material.

BEST MODE FOR CARRYING OUT THE INVENTION

The methods of the present invention comprise providing source materialssuch as flexible sheets of graphite material. The source materialstypically comprise graphite, a crystalline form of carbon comprisingatoms covalently bonded in flat layered planes with weaker bonds betweenthe planes. In obtaining source materials such as the above flexiblesheets of graphite, particles of graphite, such as natural graphiteflake, are typically treated with an intercalant of, e.g. a solution ofsulfuric and nitric acid, the crystal structure of the graphite reactsto form a compound of graphite and the intercalant. The treatedparticles of graphite are hereafter referred to as “particles ofintercalated graphite.” Upon exposure to high temperature, theintercalant within the graphite decomposes and volatilizes, causing theparticles of intercalated graphite to expand in dimension as much asabout 80 or more times its original volume in an accordion-like fashionin the “c” direction, i.e. in the direction perpendicular to thecrystalline planes of the graphite. The exfoliated graphite particlesare vermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into flexible sheets that,unlike the original graphite flakes, can be formed and cut into variousshapes and provided with small transverse openings by deformingmechanical impact.

Graphite starting materials for the flexible sheets suitable for use inthe present invention include highly graphitic carbonaceous materialscapable of intercalating organic and inorganic acids as well as halogensand then expanding when exposed to heat. These highly graphiticcarbonaceous materials most preferably have a degree of graphitizationof about 1.0. As used in this disclosure, the term “degree ofgraphitization” refers to the value g according to the formula:$g = \frac{3.45 - {d(002)}}{0.095}$where d(002) is the spacing between the graphitic layers of the carbonsin the crystal structure measured in Angstrom units. The spacing dbetween graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as carbons prepared bychemical vapor deposition and the like. Natural graphite is mostpreferred.

The graphite starting materials for the flexible sheets used in thepresent invention may contain non-carbon components so long as thecrystal structure of the starting materials maintains the requireddegree of graphitization and they are capable of exfoliation. Generally,any carbon-containing material, the crystal structure of which possessesthe required degree of graphitization and which can be exfoliated, issuitable for use with the present invention. Such graphite preferablyhas an ash content of less than twenty weight percent. More preferably,the graphite employed for the present invention will have a purity of atleast about 94%. In the most preferred embodiment, the graphite employedwill have a purity of at least about 99%.

A common method for manufacturing graphite sheet is described by Shaneet al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 20 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is a solution of amixture of sulfuric acid, or sulfuric acid and phosphoric acid, and anoxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solution maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

The quantity of intercalation solution may range from about 20 to about150 pph and more typically about 50 to about 120 pph. After the flakesare intercalated, any excess solution is drained from the flakes and theflakes are water-washed.

Alternatively, the quantity of the intercalation solution may be limitedto between about 10 and about 50 pph, which permits the washing step tobe eliminated as taught and described in U.S. Pat. No. 4,895,713, thedisclosure of which is also herein incorporated by reference.

The particles of graphite flake treated with intercalation solution canoptionally be contacted, e.g. by blending, with a reducing organic agentselected from alcohols, sugars, aldehydes and esters which are reactivewith the surface film of oxidizing intercalating solution attemperatures in the range of 25° C. and 125° C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1, 10 decanediol, decylaldehyde, 1-propanol, 1,3propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose,lactose, sucrose, potato starch, ethylene glycol monostearate,diethylene glycol dibenzoate, propylene glycol monostearate, glycerolmonostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodiumlignosulfate. The amount of organic reducing agent is suitably fromabout 0.5 to 4% by weight of the particles of graphite flake.

The use of an expansion aid applied prior to, during or immediatelyafter intercalation can also provide improvements. Among theseimprovements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

Representative examples of saturated aliphatic carboxylic acids areacids such as those of the formula H(CH₂)_(n)COOH wherein n is a numberof from 0 to about 5, including formic, acetic, propionic, butyric,pentanoic, hexanoic, and the like. In place of the carboxylic acids, theanhydrides or reactive carboxylic acid derivatives such as alkyl esterscan also be employed. Representative of alkyl esters are methyl formateand ethyl formate. Sulfuric acid, nitric acid and other known aqueousintercalants have the ability to decompose formic acid, ultimately towater and carbon dioxide. Because of this, formic acid and othersensitive expansion aids are advantageously contacted with the graphiteflake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids iscitric acid.

The intercalation solution will be aqueous and will preferably containan amount of expansion aid of from about 1 to 10%, the amount beingeffective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

After intercalating the graphite flake, and following the blending ofthe intercalant coated intercalated graphite flake with the organicreducing agent, the blend is exposed to temperatures in the range of 25°to 125° C. to promote reaction of the reducing agent and intercalantcoating. The heating period is up to about 20 hours, with shorterheating periods, e.g., at least about 10 minutes, for highertemperatures in the above-noted range. Times of one-half hour or less,e.g., on the order of 10 to 25 minutes, can be employed at the highertemperatures.

The thus treated particles of graphite are sometimes referred to as“particles of intercalated graphite.” Upon exposure to high temperature,e.g. temperatures of at least about 160° C. and especially about 700° C.to 1000° C. and higher, the particles of intercalated graphite expand asmuch as about 80 to 1000 or more times their original volume in anaccordion-like fashion in the c-direction, i.e. in the directionperpendicular to the crystalline planes of the constituent graphiteparticles. The expanded, i.e. exfoliated, graphite particles arevermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into flexible sheets that,unlike the original graphite flakes, can be formed and cut into variousshapes and provided with small transverse openings by deformingmechanical impact as hereinafter described.

Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed, e.g. by roll-pressing, to athickness of about 0.075 mm to 3.75 mm and a typical density of about0.1 to 1.5 grams per cubic centimeter (g/cc). From about 1.5-30% byweight of ceramic additives can be blended with the intercalatedgraphite flakes as described in U.S. Pat. No. 5,902,762 (which isincorporated herein by reference) to provide enhanced resin impregnationin the final flexible graphite product. The additives include ceramicfiber particles having a length of about 0.15 to 1.5 millimeters. Thewidth of the particles is suitably from about 0.04 to 0.004 mm. Theceramic fiber particles are non-reactive and non-adhering to graphiteand are stable at temperatures up to about 1100° C., preferably about1400° C. or higher. Suitable ceramic fiber particles are formed ofmacerated quartz glass fibers, carbon and graphite fibers, zirconia,boron nitride, silicon carbide and magnesia fibers, naturally occurringmineral fibers such as calcium metasilicate fibers, calcium aluminumsilicate fibers, aluminum oxide fibers and the like.

The flexible graphite sheet can also, at times, be advantageouslytreated with resin and the absorbed resin, after curing, enhances themoisture resistance and handling strength, i.e. stiffness, of theflexible graphite sheet as well as “fixing” the morphology of the sheet.Suitable resin content is preferably at least about 5% by weight, morepreferably about 10 to 35% by weight, and suitably up to about 60% byweight. Resins found especially useful in the practice of the presentinvention include acrylic-, epoxy- and phenolic-based resin systems, ormixtures thereof. Suitable epoxy resin systems include those based ondiglycidyl ether or bisphenol A (DGEBA) and other multifunctional resinsystems; phenolic resins that can be employed include resole and novolacphenolics.

Nonetheless, the graphite sheet as prepared above is cut and trimmed toform the desired articles. The methods of the present invention may usethe above-described graphite sheets including the trimmed portions. Morespecifically, the process of the present invention may use theabove-described graphite sheets including the trimmed portions atvarious stages of completeness, as discussed below.

An embodiment of the present invention includes a method ofmanufacturing flexible sheets of expanded graphite material fromrecycled materials, comprising providing source materials in the form offlexible sheets of expanded graphite such as the sheets discussed above.The source material may be sheets or trimmed portions of sheets thathave been compressed with, for example, pre-calendering rolls, but havenot yet been impregnated with resin. Furthermore, the source materialmay be sheets or trimmed portions of sheets that have been impregnatedwith resin, but not yet cured, or sheets or trimmed portions of sheetsthat have been impregnated with resin and cured. The source material mayalso be recycled flexible graphite PEM fuel cell components such as flowfield plates or electrodes. These sources are generally shown in FIG. 2.Each of the various sources of graphite may be used as is or blendedwith natural graphite flakes.

Once the source material of flexible graphite sheets is available, itcan then be comminuted by known processes or devices, such as a jetmill, air mill, blender, etc. to produce particles. Preferably, amajority of the particles have a diameter such that they will passthrough 20 U.S. mesh; more preferably a major portion (greater thanabout 20%, most preferably greater than about 50%) will not pass through80 U.S. mesh. Most preferably the particles have a particle size of nogreater than about 20 mesh. It may be desirable to cool the flexiblegraphite sheet when it is resin-impregnated as it is being comminuted toavoid heat damage to the resin system during the comminution process.

The size of the comminuted particles may be chosen so as to balancemachinability and formability of the graphite article with the thermalcharacteristics desired. Thus, smaller particles will result in agraphite article which is easier to machine and/or form, whereas largerparticles will result in a graphite article having higher anisotropy,and, therefore, greater in-plane electrical and thermal conductivity.

If the source material has been resin impregnated, then preferably theresin is removed from the particles. Details of the resin removal arefurther described below.

Once the source material is comminuted, and any resin is removed, it isthen re-expanded. The re-expansion may occur by using the intercalationand exfoliation process described above and those described in U.S. Pat.No. 3,404,061 to Shane et al. and U.S. Pat. No. 4,895,713 to Greinke etal.

Typically, after intercalation the particles are exfoliated by heatingthe intercalated particles in a furnace. During this exfoliation step,intercalated natural graphite flakes may be added to the recycledintercalated particles.

Preferably, during the re-expansion step the particles are expanded tohave a specific volume in the range of at least about 100 cc/g and up toabout 350 cc/g or greater.

Finally, after the re-expansion step, the re-expanded particles may beformed into a mat using, for example, the process for forming a graphitesheet discussed above. This process is generally shown in the flow chartof FIG. 1.

This newly formed flexible graphite sheet may be further formed into thecomponents of a PEM fuel cell discussed above. As stated above, in thisembodiment the source materials may include uncured resin impregnatedexpanded graphite sheets and cured resin impregnated expanded graphitesheets.

The methods of the present invention may further comprise calenderingthe mat into a flexible sheet of expanded graphite material. Thespecific calendering method is not known to be critical, and any methodknown in the art is suitable.

If the starting material has been impregnated with a resin, an importantembodiment of the method of the present invention is removing at leastpart of the resin from the particles. This removal step should occurbetween the comminuting step and the re-expanding step.

In one embodiment, the removing step includes heating the resincontaining regrind particles, such as over an open flame. Morespecifically, the impregnated resin may be heated to a temperature of atleast about 250° C. to effect resin removal. During this heating stepcare should be taken to avoid flashing of the resin decompositionproducts; this can be done by careful heating in air or by heating in aninert atmosphere. Preferably, the heating should be in the range of fromabout 400° C. to about 800° C. for a time in the range of from at leastabout 10 and up to about 150 minutes or longer.

Additionally, the resin removal step may result in increased tensilestrength of the resulting sheet produced from the calendering step ascompared to a similar method in which the resin is not removed.Therefore, when the process of the present invention further comprisescalendering the mat into a flexible sheet of expanded graphite material,the resin removing step results in increased tensile strength of thesheet created in the calendering step, as compared to a similar methodin which the resin is not removed. More specifically, the sheet createdin the calendering step can have a tensile strength of at least about300 psi.

The resin removal step may also be advantageous because during theexpansion step. (i.e., intercalation and exfoliation), when the resin ismixed with the intercalation chemicals, it may in certain instancescreate toxic byproducts.

Thus, by removing the resin before the expansion step a superior productis obtained such as the increased strength characteristics discussedabove. The increased strength characteristics are a result of in partbecause of increased expansion. With the resin present in the particles,expansion may be restricted.

In addition to strength characteristics and environmental concerns,resin may be removed prior to intercalation in view of concerns aboutthe resin possibly creating a run away exothermic reaction with theacid.

In view of the above, preferably a majority of the resin is removed.More preferably, greater than about 75% of the resin is removed. Mostpreferably, greater than 99% of the resin is removed.

Exemplary resin removal temperatures are shown in FIGS. 3-5. FIGS. 3-5are thermogravimetric analysis charts showing the temperatures at whichepoxy resin may be removed from the graphite source material. FIG. 3 isan informational chart that demonstrates a 100% graphite sample (i.e.,no resin added) of a flexible graphite sheet heated at 10° C./min inair, begins losing weight at approximately 600° C. Therefore, inrecycling flow field plates, for example, the resin is preferablyremoved at less than 600° C. in order to leave the graphite intact sothat it may be recycled.

FIG. 4 is the same type of analysis as FIG. 3, with the exception thatthe source material contains 37.5% epoxy and 62.5% graphite. Weight lossbegins at approximately 250° C. and all the resin is removed atapproximately 600° C. Therefore, in a preferred embodiment of theinvention, the resin is removed at a temperature from 250° C. to about600° C.

FIG. 5 demonstrates the weight loss for a similar material as thematerial of FIG. 4, however as shown in FIG. 5 the material is heated to500° C. at 10° C./min and held for 150 minutes at 500° C. This treatmentshould be sufficient to burn off virtually all the resin and leave thegraphite intact for recycling purposes.

All cited patents and publications referred to in this application areincorporated by reference.

The invention thus being described, it will be obvious that it may bevaried in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the present invention and allsuch modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A method of manufacturing flexible sheets of expanded graphitematerial from recycled materials, comprising: (a) providing sourcematerials in the form of flexible sheets of expanded graphite; (b)comminuting the source materials into particles; (c) re-expanding theparticles; and (d) preparing a mat from the re-expanded particles. 2.The method of claim 1, further comprising: compressing the mat into aflexible sheet of expanded graphite material.
 3. The method of claim 1,wherein: in step (a) the source materials are selected from the groupconsisting of unimpregnated expanded graphite sheets, uncured resinimpregnated expanded graphite sheets and cured resin impregnatedgraphite sheets.
 4. The method of claim 1, wherein: in step (a) thesource materials include resin impregnated expanded graphite sheets. 5.The method of claim 4, further comprising: between steps (b) and (c),removing at least part of the resin from the particles.
 6. The method ofclaim 5, wherein: the removing step includes heating the particles,while avoiding flashing of the resin decomposition products from theparticles.
 7. The method of claim 5, wherein: the removing step includesheating the particles at less than about 600° C.
 8. The method of claim5, wherein: the removing step includes heating the particles to atemperature in a range of from about 250° C. to about 600° C.
 9. Themethod of claim 5, wherein: the removing step includes heating theparticles at about 500° C. for about 150 minutes.
 10. The method ofclaim 5, further comprising: compressing the mat into a flexible sheetof expanded graphite material; and wherein the removing step results inincreased tensile strength of the sheet created in the compressing step,as compared to a similar method in which the resin is not removed. 11.The method of claim 5, wherein the removing step further comprises:removing greater than 75% of the resin.
 12. The method of claim 1,wherein: step (c) includes steps of intercalating the particles andheating the intercalated particles.
 13. The method of claim 12, wherein:in step (c), the particles are expanded to have a specific volume of atleast about 100 cc/g.
 14. The method of claim 1, wherein: in step (b),the particles have a particle size of no greater than about 20 mesh. 15.The method of claim 1, further comprising: impregnating the mat withresin; compressing the impregnated mat into a flexible sheet of expandedgraphite material; and embossing and curing the sheet.
 16. The method ofclaim 1, further comprising: blending the particles with expandablenatural graphite flakes.
 17. The method of claim 16, wherein: step (c)includes steps of intercalating the particles and exfoliating theintercalated particles by heating the intercalated particles in afurnace; and the blending step occurs in the furnace during theexfoliation process.
 18. A material suitable for the construction of acomponent of a fuel cell, manufactured by the process of claim
 1. 19. Aprocess of manufacturing a graphite material comprising: (a) grinding aresin impregnated graphite material into particles; (b) removing atleast part of the resin from the particles; and (c) expanding the resinremoved particles.
 20. The process of claim 19, further comprising:after step (c), forming the expanded resin removed particles, into aflexible sheet of graphite material.
 21. The process of claim 19,wherein: step (c) includes an intercalation process followed byexfoliation of the intercalated particles in a furnace.
 22. The processof claim 21, further comprising: blending expandable natural graphiteflake material with the intercalated particles in the furnace during theexfoliation process.
 23. The method of claim 19, wherein step (b),further comprises: heating the particles, while avoiding flashing oftheresin decomposition products from the particles.
 24. The method ofclaim 19, wherein: the removing step includes heating the particles atless than about 600° C.
 25. The method of claim 19, wherein: theremoving step includes heating the particles to a temperature in a rangeof from about 300° C. to about 600° C.
 26. The method of claim 19,wherein: the removing step includes heating the particles in the rangeof from about 400° C. to about 800° C. for a time in the range of fromabout 10 to about 150 minutes.
 27. The method of claim 19, wherein: instep (a), the particles have a particle size of no greater than about 20mesh.
 28. The method of claim 19, wherein: in step (b), greater than 75%of the resin is removed.